1. Field of the Invention
The present invention relates to upward-joining-noise decreasing methods and apparatuses, and more particularly to an upward-joining-noise decreasing method and an apparatus for decreasing noise of upward signals used in two-way communication services of a CATV (cable television) transmission system.
2. Description of the Related Art
FIG. 1 is a diagram showing a conventional frequency spectrum of some ordinary upward signals used in two-way communication services of a CATV transmission system. As may be seen from this diagram, an upward-date-communication signal, which may be used in the two-way communication services such as the Internet, is transmitted within a band ranging from 30 to 36 MHz, and an upward-video signal, which may be used in a television broadcasting or the like, is transmitted within a band ranging between 36 to 42 MHz. In addition, a pilot signal, which serves to automatically adjust the gain of a transmission-line amplifier as a reference signal, is transmitted within a narrow band centered on approximately 48 MHz, and an STM (status monitoring) signal, which serves to monitor the state of a transmission-line amplifier at a center device as a response signal of a status monitor system, is transmitted within a band ranging from 54 to 55 MHz.
FIG. 2 is a diagram showing an example of the conventional CATV transmission system. As may be seen from this diagram, there are a plurality of CATV transmission lines 10A, 10B, . . . , 10X, each of which may, if necessary, be provided with a plurality of transmission-line amplifiers 12 which are respectively coupled to a plurality of subscriber terminals 14. Upward signals transmitted by the respective transmission lines 10A, 10B, . . . ,10X are mixed at a head-end device 16 in which a mixed signal is generated therefrom and then supplied to a center device 18.
In general, the upward-video signal for television broadcasting is transmitted to the center device 18 from a transmitting terminal (upward television modulator) provided on a CATV transmission line, for example, 10A. In this case, since communications between the center device 18 and the transmission terminal is xe2x80x9cone-to-onexe2x80x9d communications, it can be performed by using only one transmission line connected therebetween. For this reason, the influence of noise upon upward signals from the transmission line is minimal and ignored.
In contrast, in a case of the two-way data communication system, in order to perform communications from subscriber terminals provided in subscriber houses, the system should be configured such that the center device 18 (one device) can cope with a number N of the subscriber terminals 14 (N devices), namely, an xe2x80x9cone-to-Nxe2x80x9d system. In this case, since upward signals from all of the transmission lines 10A, 10B, . . . , 10X are concentrated on the center device 18, noise contained in these upward signals are all concentrated thereon as well.
Thereby, even though the noise coming from one transmission line may be too small to be worried about, noise coming from a plurality of the transmission lines can become upward-joining noise with a level high enough to interfere with communications on these transmission lines. For this reason, there is a problem that if noise coming from one transmission line is increased during communications on this transmission line, it may interfere with communications of all the other transmission lines. Further, in general, the lower the frequency band becomes, the larger the upward-joining noise contained therein becomes. Therefore, the frequency band to be used in communications is usually restricted to be medium or high.
In order to solve the above-mentioned problem, conventionally, when noise from a certain transmission line increases, the center device 18 measures noise from each transmission line so as to identify from which transmission line the noise has increased.
Also, a transmission-line-correction method, as a method for suppressing noise generated on the transmission lines, is provided in which a status monitoring device (STM device) is used to close a gate corresponding to a transmission line on which noise is being generated so as to interrupt upward signals therefrom. The transmission line correction method determines the locations of noise sources on a specified transmission line so as to correct the specified transmission line.
The method using the STM device may be understood with reference to FIG. 3. FIG. 3 is a diagram showing a conventional CATV transmission system using the STM device. As may be seen from this diagram, the transmission lines 10A, 10B and 10C have a transmission-line amplifier 20 provided at a junction thereof, which includes a gate with respect to these transmission lines 10A through 10C. And a CATV center has an STM device 24 and the center device 18 which are coupled to the head-end apparatus 16. When the STM device 24 determines that the noise from the line 10A is increased according to a status signal supplied by the transmission-line amplifier 20, it controls the gate thereof so that the gate is closed only to the transmission line 10A and opened for the transmission lines 10B and 10C.
Recently, a communication establishing method has been provided in which a tap-off-connection portion is provided on the side of each subscriber terminal with a gate which is closed before communications. When the subscriber terminal transmits a specific signal for communications to the tap-off-connection portion, the tap-off-connection receives the specific signal and then switches the gate ON, and communication from the subscriber terminal to the center device is thus established. Accordingly, seen from the center device, the gate of only one transmission line is switched ON at any time, and thereby an increase in noise due to a mixture of upward signals from a plurality of transmission lines can be prevented.
Further, a conventional system is provided which comprises a switching device provided immediately before the center device so as to communicate with subscriber terminals while switching transmission lines connected between the center device and the subscriber terminals. According to this system, when the center device communicates with the subscriber terminals by means of the switching device, only one transmission line can be coupled to the center device at a time, therefore preventing noise coming from a plurality of transmission lines being added together and then being supplied to the center device.
However, it takes much time and labor to determine on which transmission line the noise is increased by means of the center device. Herein, if the noise on a certain transmission line increases due to its deterioration with age, it is easy to determine the transmission line by measuring habitual noise. But, if the noise thereon is increased due to a transient cause, it is difficult and time-consuming to determine the transmission line by the above-mentioned conventional method.
With respect to the transmission-line-correction method, there is a problem that communication services on a transmission line to be corrected must be interrupted for a given time during the correction. Also, with respect to the method using the STM device, there is a problem that the two-way communication is available for the lines 10B and 10C and is unavailable for the line 10A which has to be corrected separately.
With respect to the method in which the gate is provided in the tap-off-connection portion, since the gate has to be opened during communications, it is necessary to transmit/receive a gate-ON signal besides information about the communications, that is, additional information about the gate-ON signal has to be processed. For this reason, it takes time to turn the gate ON/OFF and this may result in degradation in the communication of the upward signals. Further, there is a problem that the upward-joining noise generated on each transmission line interferes with the communications until the transmission line is corrected.
With respect to the method in which the switching device is provided immediately before the center device, in fact, a level of the noise cannot be decreased by switching the transmission lines only one of which is coupled to the center device at a time. For this reason, in a case where large noise is generated on the transmission line which is coupled to the center device, the influence of the noise upon communication services performed on the transmission line cannot be prevented and results in a deterioration of the communication services.
It is a general object of the present invention to provide an upward-joining-noise decreasing method and apparatus, in which the above disadvantages are eliminated.
Another and more specific object of the present invention is to provide a method for decreasing upward-joining-noise in a two-way communication system connected by a plurality of transmission lines between a center device and a plurality of subscriber terminals, said upward-joining-noise decreasing method comprising the steps of:
in an upward-signal-inputting position of said center device,
(a) measuring a noise level of each of a plurality of upward signals supplied from the plurality of transmission lines;
(b) determining whether said measured noise level of said each upward signal is low enough or not;
(c) controlling a level of said each upward signal based on a result obtained from said determining step (b); and
(d) adjusting said level of said each upward signal according to said controlling step (c).
Still another object of the present invention is to provide a method for decreasing upward-joining-noise in a two-way communication system connected by a plurality of transmission lines between a center device and a plurality of subscriber terminals, said upward-joining-noise decreasing method comprising the steps of:
at a plurality of transmission-line amplifiers provided on the plurality of transmission lines,
(a) measuring a noise level of each of a plurality of upward signals supplied from the plurality of transmission lines and the plurality of subscriber terminals;
(b) determining whether said measured noise level of said each upward signal is low enough or not;
(c) controlling a level of said each upward signal based on a result obtained from said determining step (b); and
(d) adjusting said level of said each upward signal according to said controlling step (c).
Still another object of the present invention is to provide an upward-joining-noise decreasing apparatus used in a two-way communication system connected by a plurality of transmission lines between a center device and a plurality of subscriber terminals, said upward-joining-noise decreasing apparatus, which is mounted in an upward-signal-inputting position of said center device, comprising:
a plurality of level measuring portions, each for measuring a noise level of each of a plurality of upward signals supplied from the plurality of transmission lines;
a level determining portion for determining whether said measured noise level of said each upward signal is low enough or not;
a level control portion for controlling a level of said each upward signal based on a result obtained from said level determining portion; and
a plurality of level adjusting portions, each for adjusting said level of said each upward signal according to said level control portion.
Still another object of the present invention is to provide an upward-joining-noise decreasing apparatus used in a two-way communication system connected by a plurality of transmission lines between a center device and a plurality of subscriber terminals, said upward-joining-noise decreasing apparatus, which is provided in each of a plurality of transmission-line amplifiers provided on the plurality of transmission lines, comprising:
a plurality of level measuring portions, each for measuring a noise level of each of a plurality of upward signals supplied from the plurality of transmission lines and the plurality of subscriber terminals;
a level determining portion for determining whether said measured noise level of said each upward signal is low enough or not;
a level control portion for controlling a level of said each upward signal based on a result obtained from said level determining portion; and
a plurality of level adjusting portions, each for adjusting said level of said each upward signal according to said level control portion.